1. Field of Invention
The present invention relates generally to the fields of both analog and digital circuits for signal modulation. More specifically, the present invention relates to a method and an apparatus for dynamically modulating periodic signal from PWM to reduce measurable radiated and/or conducted electromagnetic interference (EMI).
2. Description of the Related Art
Switch Mode Power Supplies (SMPS) tend to generate EMI due to rapid switching of current and voltage. The spectral components of the EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of a clock. Accordingly, many regulatory agencies, such as the FCC in the United States, have established testing procedures and maximum allowable emissions for such products. For example, the Commission Electrotechnique Internationale (Comite International Special Des Perturbations Radioelectriques (C.I.S.P.R.)) has guidelines establishing measurement equipment and techniques for determining compliance with regulations. In order to comply with such regulatory limits on EMI emissions, costly suppression measures or extensive shielding may be required. Some techniques used to modify the EMI spectrum have also been proposed, including frequency modulation or frequency hopping. Information relevant to attempts to address these problems can be found in the following publications and U.S. patents: Virginia Power Electronics Center VPED Tenth Annual Power Electronics Seminar, pp. 129-136, Sep. 20-22, 1992 published by Lin et al; U.S. Pat. Nos. 5,488,627 and 5,867,524 issued to Keith B. Hardin et al.; U.S. Pat. Nos. 7,203,079 and 7,391,628 issued to Da-Yung Yang et al.; and U.S. Pat. No. 7,304,522 issued to Jong-hoon Kim et al.
However, each one of these references suffers from one or more of the following disadvantages. An EMI filter causes power consumption and increases the cost and size of the power supply. Frequency modulation alone is not necessarily able to reduce EMI enough to meet the regulation requirements, such as Quasi-Peak measurement. And, although spread spectrum modulation with fixed modulation depth may smear the peak EMI energy out into additional sidebands, it is not adequate to address the difficulties associated with reducing the spectral amplitudes of EMI in lower harmonics, such as the second and third harmonics. For the foregoing reasons, there is a need to reduce radiated and/or conducted EMI without the above problems.